The objective of the study is to perform a double-blinded study to test the ability of aminoglycoside to induce read-through of stop mutations and restore functional CFTR protein in the nasal epithelium. Cystic fibrosis patients with the stop mutation G542X, R553X, W1282X or R1162X will be recruited. Gentamicin will be applied to the nasal epithelium of one nostril then the contralateral nostril will receive dilulent alone. One assay will be used to assess restoration of funcitonal CFTR. This assay is nasal potential difference measurement.